1. Field of the Invention
The present invention relates to a display device which mitigates reduced contrast (bright-room contrast) due to external light when a display screen is viewed in a bright room in order to reduce blurred images, and more particularly, to a front plate which is integrally or separately arranged on a front surface of the display device to improve the contrast of viewed images, and a display device using the same.
2. Description of the Related Art
Conventionally, for improving the contrast in a bright room (hereinafter called the “bright-room contrast) of a display device using a display panel having selfluminous pixels, arranged in matrix, which emit light in accordance with the Lambert distribution, the transmittance τ of a front plate arranged integrally or separately on a front surface of a display panel, such as a plasma display panel (hereinafter abbreviated as “PDP”) or the like, for displaying an image is set in a range of approximately 35 to 70%, such that external light is attenuated while room light (hereinafter called the “external light”) incident on the front plate is reflected back by an opposite surface of the front plate or the display panel (i.e., while the light goes to the front plate and back twice) to increase the ratio of output light from the display panel to the external light, thereby improving the contrast. Such a technique is disclosed, for example, in page 2, right column, paragraph number 0009 of JP-A-2000-206932.
Description will be made qualitatively with reference to a diagram shown in FIG. 10. In FIG. 10, output light B from a display panel 2 reduces its value to B×τ (τ: transmittance) while it passes through a front plate 200. On the other hand, since external light b goes to and back from the front plate 200, the transmittance of which is τ, its magnitude changes to b×τ×τ during the reciprocal travel. When the transmittance τ is 0.5, the external light largely attenuates, and reduces to 0.25 times as high as when there is no front plate 200 for adjusting the transmittance. For this reason, a high contrast image can be viewed in a bright room.
Other means for improving the bright-room contrast include a technique for applying an anti-reflection film on an external light incident surface of the front plate 200 for preventing the external light from reflecting, a technique for forming miniature ruggedness to irregularly reflect external light, and the like. Either of them improves the bright-room contrast by preventing the glaring of external light. These techniques are disclosed, for example, in FIG. 4 of JP-A-2002-83548, JP-A-6-187924, JP-A-10-283939, and the like.
An anti-reflection film 210 in FIG. 10 prevents the glaring due to external light as mentioned above. The anti-reflection film 210 acts to largely reduce reflection of external light on the surface of the front plate 200. However, the external light which transmits the front plate 200 is reflected back from the opposite surface of the front plate 200 or the display panel, and exits from the front plate toward a viewer. In this event, if the external light transmitting the front plate 200 does not attenuate while it is reflected back, the external light reduces the bright-room contrast. Generally, the anti-reflection film is used in combination with an improving means for reducing the transmittance of the front plate, such that the external light transmitting the front plate is absorbed by the front plate with the transmittance τ to attenuate as mentioned above.
A further means for improving the contrast in a bright room involves forming an exit surface of a transmission type screen with conical protrusions, for example, circular truncated cones similar to wedge-shaped protrusions used on wall surfaces of an acoustic anechoic chamber, a radio anechoic chamber and the like for absorbing sound and radio waves without reflection, for example, as disclosed in JP-A-6-194741, JP-A-2003-50307, and the like.
Since the present invention is deeply related to JP-A-6-194741 and JP-A-2003-50307, reference is made to FIGS. 1 and 2 of JP-A-6-194741 for describing the foregoing technique in detail.
In FIGS. 1 and 2 of JP-A-6-194741, as external light 23 is incident on an inclined portions 14 of a convex protrusion 12, the external light 23 repeats reflections multiple times as shown (indicated by dotted lines) between adjacent inclined portions 14 to attenuate.
On the other hand, projected light 21 from the back of a screen propagates within the convex protrusions 12 nearly in a total reflection condition, and is outputted from a peak 13 with less attenuation. Specifically, the reflection of the external light 23 can be reduced while maintaining the projected light 21 from the back of the screen at substantially consistent brightness without largely attenuating the projected light 21, so that the bright-room contrast can be satisfactorily improved.
In addition, the inclined portions 14 may be coated with a light absorption layer 15 or the like to absorb reflected light, resulting in a higher contrast ratio.
As described above, the technique using the convex protrusions demonstrates significant effects for improving the contrast of images in a bright room.